Serum IgG antibodies to teichoic acid and alpha toxin from Staphylococcus aureus were measured in 62 patients with cystic fibrosis by enzyme linked immunosorbent assays. The patients were followed up for 12-24 months in a prospective study. Raised titres were found exclusively in patients chronically colonised with S aureus. Patients colonised with both S aureus and Pseudomonas aeruginosa had significantly higher titres against teichoic acid than those carrying S aureus alone. Titres were significantly higher when there were clinical signs of low grade infection in the patients chronically colonised with S aureus alone, and in those with both S aureus and P aeruginosa. Significant reduction in titres occurred after antimicrobial treatment given either orally or intravenously in patients with normal erythrocyte sedimentation rates and white cell counts. Measurement of staphylococcal antibody titres may be valuable in monitoring pulmonary infection and antimicrobial treatment in patients with cystic fibrosis.